The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
U.S. Pat. No. 4,953,305 discloses an electrical compass system for use in a vehicle in which a magnetic sensor, such as a flux-gate sensor, is employed in connection with a microprocessor to provide a digital display of the vehicle heading. The system described in the '305 patent provides continuous automatic calibration of the compass to correct for changes in the detected magnetic field due to the vehicle's magnetism and changes thereto as well as provides a system which, in effect, filters out magnetic aberrations caused by travel within an area where severe electromagnetic interference may temporarily occur.
The magnetic sensors are optimally located in the upper windshield area of a vehicle, preferably near the vehicle roof and remote from large current carrying conductors in the vehicle's electrical system which may otherwise cause magnetic disturbance which adversely affects compass operation. One system incorporated a sensor located in their rear package tray area of the vehicle and operation of the rear window defroster which draws significant current caused undesirable interference with the compass operation. To correct for this condition which occurs only on an infrequent basis, the compass included a sensing input for determining when the rear window defroster was on and provided a correction signal to the compass microprocessor to cancel out the magnetic field disturbance caused by operation of the rear window defroster. It is preferable however to mount the magnetic field sensor in areas remote from such electrical disturbance as, for example, taught in U.S. Pat. No. 4,505,054. Vehicle design however, does not always accommodate such a mounting system. In view of the desirability of mounting the magnetic sensor on a circuit board which includes the remaining compass elements, if it is desired to have the compass display in, for example, the instrument panel, the sensor mounted on the board with the display must of necessity also be mounted in the instrument panel.
The instrument panel area of a vehicle is generally considered an unsuitable environment for an electronic compass sensor and has heretofore been avoided. It has been discovered, for example, that current through the fan motor for the vehicle's HVAC system can vary the ambient magnetic field from 10 to 40 milligauss for each of the detection axes of the compass sensor as a function of the fan speed setting. As can be appreciated with a higher fan speed, a higher magnitude of DC magnetic field disturbance is introduced in the area of the sensor if mounted in the instrument panel area adjacent a conductor supplying current to the fan motor.